Existing communication systems suffer from a number of effects which act to degrade the quality of communication between a transmitting unit and a receiving unit within the communication system. In particular, in a radio telephone network for example, which comprises a plurality of mobile stations or radio telephones communicating with a base station connected to other base stations in the radio telephone network, both the radio telephone and the base station are relatively close to ground level. Since the base station and radio telephone are close to ground level obstacles such as buildings, walls, cars and people inhibit direct line of sight communication between them, especially in urban areas. Thus, they typically communicate between each other by reflected or diffracted radio wave signals. Due to the multiple reflections and diffractions the r.f. power received by a radio telephone or a base station is at a much lower level than would be expected from the inverse square law if direct line of sight communication was possible. Typically, the power loss is of the form d.sup.-a where d is the distance between the transmitting and receiving stations and a lies between 3 and 4. This power loss is known as path loss.
The problem of path loss has been addressed in known radio telephone systems by the base stations monitoring the strength of signals received from various radio telephones communicating therewith (to form a received signal strength indicator RSSI signal), and from time to time issuing a request over the air for an individual radio telephone to increase or decrease its transmitting power. The radio telephone responds by adjusting the gain of its transmitting amplifier which is typically under microprocessor control. Generally, the amplifier is operable at one of a plurality of predetermined output power levels which are selected automatically in response to the request from the base station for a change in the level of the output power. Typically, the power levels are defined in the radio telephone system specification. For each power level a nominal value is specified together with a permitted tolerance range.
In addition to the normal fading there is another form of fading known as Rayleigh fading. This type of fading is a short term fading and is characterised by rapid variations in the r.f. power level of a signal received by a radio telephone or base station. It is caused by the multiple signal paths arising from the reflections and diffractions forming a quasi-stationary standing wave pattern with nulls at approximately half wavelength intervals of the signal frequency. As a user moves through their environment, they move through the nulls.
The effect of the periodic nulls in received signal power due to Rayleigh fading is that transmitted data may be lost thereby introducing errors into the transmission. In order to ensure that there is sufficient integrity in the radio telephone network redundant data has to be sent such as error-correcting codes. This results in a reduced information or data handling capacity for the network. Additionally, the multiple signal paths introduce time delays between signals incident at a particular radio telephone which causes inter-symbol interference. This is a particular problem in communication systems having relatively high data rates e.g. where ##EQU1##
The problems of Rayleigh fading have been addressed by using a technique known as Slow Frequency Hopping (SFH) or Frequency Hopping. In this technique, the carrier frequency of a particular communication channel is discontinuously changed between discrete carrier frequencies of a set of defined carrier frequencies. Since the Rayleigh fading of signals at different frequencies is not the same, and becomes increasingly different as the difference between the frequencies increases, frequency hopping for a particular communication channel substantially reduces the effects of Rayleigh fading for that communication channel effectively transforming errors due to Rayleigh fading into widely spread random errors. Another advantage is that co-channel interference from other cells is reduced.
Such a technique is known from the GSM system for cellular radio telephony, where the sequence of data bursts making up a particular communication channel or Traffic Channel (TCH) are cyclically assigned to different frequencies by the base station handling that communication channel. Additionally, a technique known as interleaving is employed in the GSM system. This involves jumbling up data to be transmitted such that normally adjacent groups of data are transmitted at different times, and de-interleaving the transmitted signal at the receiver.
In a system having relatively low data rates, e.g. .gtoreq.25k symbol/s, the periodic variation in the signal strength is the main problem. Such a system is typically referred to as suffering from non-frequency selective or "flat" multipath Rayleigh fading. The Japanese PDC system is such a system. It is known to utilize two or more antennas in a mobile terminal to reduce the effects of flat Rayleigh fading. This is commonly referred to as antenna diversity. Respective antennas are selected for use based on a comparison of various criteria indicative of the quality of signals received by the antennas.
A number of methods for determining the criteria for selecting an antenna are known in the art. For example, European patent application number 0 318 665 describes the selection of antennas based on a received signal strength indication (RSSI) signal in the context of a time division multiplexed radio network. In such a system, the antenna receiving the signal of greatest strength is selected. If the signal from the selected antenna falls below a threshold then another antenna is selected. A further method of selecting antennas is disclosed in European patent application number 0 454 585, which describes a method of predicting which antenna will receive a signal having the greatest quality within a given time period.
However, the known approaches to the problem of flat Rayleigh fading have their own drawbacks. For a threshold fixed at a given RSSI there is a finite probability that the signal strength will dip below the RSSI threshold thereby causing selection of another antenna. Thus, irrespective of the quality of a signal there is a minimum achievable error rate for a system due to the selection of another antenna. This gives rise to a so called error floor. For high quality signals such as those having a large signal to noise ratio (SNR) where a large fade, i.e. one which drops below the fixed RSSI threshold, does not intolerably impair the signal quality, another antenna is selected needlessly which may operate at an error rate worse than the one previously selected. Additionally, needless selection of other antennas is undesirable since there is a phase difference between signals at different antennas which results in a phase mismatch of the signals giving rise to further errors.